Electrical machines, in particular large electrical machines such as those used in power stations, should be monitored continuously in order to ensure friction-free operation and/or analyzed from time to time in order to recognize faults in time and to avoid uncontrolled failures.
EP 0 271 678 A1 proposes, for example, a device with which statements can be made on the functional reliability and functional safety of the shaft grounding or the insulation of the shaft. The device proposed therein, which is actually envisaged for reducing shaft voltages, allows statements to be made on the entire grounding conditions of the electrical machine.
If the rotating metal shaft of a machine comes into contact with or touches the metal of the housing parts surrounding it, wear occurs. With generators, considerable electrical currents can also flow via the contact points and lead to instances of overheating, erosion or even melting processes. It is correspondingly desirable to detect such instances of the metal parts coming into contact with each other during operation and to localize the touching points. In this document, the terms “touching points” and “contact points” are used interchangeably.
It is known to provide double insulation for the bearings of machines, for example for the purpose of detecting touching points, and to arrange a conductive layer between the two insulating layers. Monitoring is then carried out to ascertain whether this conductive intermediate layer has a resistive conductive connection to the shaft or to ground. If this is the case, one of the insulating layers is defective. One disadvantage of this method is the fact that the insulating layers in the bearing are only tested selectively, but not the shaft insulation per se, i.e. the insulation state of the entire shaft cannot be determined. It is therefore, for example, not possible to establish whether the shaft is in direct contact with the housing. One disadvantage of this method is, inter alia, that only the insulating layers in the bearing are tested, but not the shaft insulation per se. It is not possible to establish whether the shaft is in contact with, for example, the housing.
Methods also exist in which currents through the shaft are measured by means of uniform-field coils. Here too, contact points between the shaft and the housing or ground are not detected directly, but only the indirect consequences of these contacts.
EP 0 503 846 A2 describes a method and a device for detecting faults in the insulation between a shaft of a generator and the bearings or seals which guide said bearings. For measurement purposes, a transformer is placed around the stationary shaft and is driven with alternating current. Then, a voltage measuring device, which is grounded on one side and is connected to the shaft on the other side at a point at which a fault is suspected to be, is used to measure the voltage. Such methods allow for effective measurement of faults in the rest state, but the touching points or contact points occurring exclusively during operation cannot be found in this way.